A number of magnetic tape drive systems have been developed to provide mass data storage, for example for personal computer systems. One emerging technology providing high density storage, preferably on quarter inch magnetic tape, utilizes arcuate scanning. With this type of scanning, read and write scanner heads are mounted near the periphery of a circular planar surface and rotated thereon about an axis passing through the center of the circular surface and through the plane of a longitudinally-moving tape. In writing data on a tape, arcuate scanners produce a sequence of arcuately-shaped tracks which are transverse to the longitudinal axis of the tape.
Examples of arcuate scanning tape drives are described, for example, in: U.S. Pat. No. 2,750,449; U.S. Pat. No. 2,924,668; U.S. Pat. No. 3,320,371; U.S. Pat. No. 4,636,886; U.S. Pat. No. 4,647,993; and U.S. Pat. No. 4,731,681.
International Application WO 93/26005 to Lemke et al. discloses an example of an arcuate scanning tape drive for computer data storage. In the Lemke et al. arcuate scanning tape drive, a number of scanner heads are provided around the periphery of the circular planar surface. As the scanner rotates and the tape moves past the rotating scanner surface, the read and write heads alternately pass over the tape. The operation of the scanner is commutated or switched from "write" to "read" to alternately activate the appropriate operation through alternate ones of the scanner heads.
To effectively read and write data in arcuate tracks on a longitudinally moving tape requires (1) writing the tracks in an agreed format, position and alignment on the tape, and (2) corresponding positioning and alignment of the read heads during the read operation to locate and recover the data written on the tracks. In an arcuate scanner of the type described by Lemke et al., there are a number of variables which affect both the read operation and the write operation. These include tape speed, rotational speed of the scanner head and orientation or positioning of the scanner head with respect to the tape. Several of these variables are affected by external factors. For example, if there is some vibration of a scanner during the writing operation, it may be difficult to align the head with the data tracks during a subsequent read operation, particularly if the read operation is performed by a different scanner.
The above cited Lemke et al. document discloses a technique for controlling several of these variables during the read and write operations. In the Lemke et al. arcuate scanner, for example, the positioning of the ASHA is controlled via a servoing mechanism that senses specific servo burst information from one or more previously written tracks. For example, low frequency servo burst information is detected at either the beginning and/or at the end of a scan over the track via a transducer (e.g., a read head). Based on a corresponding low frequency servo burst signal(s), the location/alignment of the ASHA with respect to the tape/tracks is then determined. If the ASHA is not properly aligned, then adjustments are made to correct the positioning of the ASHA.
In arcuate scanning it is necessary to stay on track for recording, and on replay, it is necessary to match the pointing of the reading scanner with the pointing of the scanner that wrote the information on the tape. Both the read and write operations are therefore substantially affected by the angular position of the ASHA with respect to the tape. Misalignment often prevents accurate recovery of stored data. By relying on the low frequency servo information recorded within the tracks, the above-cited technique provides for fine-tuning and/or final alignment of the ASHA with respect to the tape. However, if the initial or a later misalignment of the ASHA with respect to the tape is significant enough, then the above-cited technique may not adequately recognize and/or correct the misalignment.